In a manufacturing process of a semiconductor device or a liquid crystal display, a plasma processing apparatus such as a plasma etching apparatus or a plasma CVD film forming apparatus is used to perform a plasma process such as an etching process or a film forming process on a processing target substrate such as a semiconductor wafer or a glass substrate.
Recently, as such a plasma processing apparatus, a radial line slot antenna (RLSA) (registered trademark) microwave plasma processing apparatus of generating high-density plasma having a low electron temperature is attracting attention (see, for example, Patent Document 1).
In the RLSA microwave plasma processing apparatus, a planar antenna provided with a multiple number of slots formed in a preset pattern is provided at an upper portion of a chamber. In this apparatus, a microwave generated from a microwave generator is radiated from the slots of the planar antenna into the chamber, which is maintained in a vacuum, through a microwave transmission plate which is provided under the planar antenna and is made of a dielectric material. A gas introduced into the chamber is excited into plasma by this microwave electric field, and the processing target substrate such as the semiconductor wafer is processed by the plasma generated as described above
Further, there is also proposed a microwave plasma processing apparatus having a microwave plasma source configured to split a microwave into plural microwaves, introduce the microwaves into a chamber via a multiple number of antenna modules having the planar antenna and combine the microwaves in a space within the chamber (Patent Document 2).
In this kind of microwave plasma processing apparatus, a tuner for automatically matching an impedance of a plasma load with a characteristic impedance of a microwave power source is required. As such a tuner, there is known a slug tuner which includes a pair of slugs movably provided within a coaxial transmission path and is configured to match the impedances by moving the slugs. A technique of automatically performing the impedance matching in this slug tuner is described in Patent Document 3.
In the technique described in Patent Document 3, a reflection coefficient of a plasma load is calculated, and the impedance matching is performed through an algorithm by which the positions of the slugs are controlled to reduce the reflection coefficient.
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-294550
Patent Document 2: International Publication No. 2008/013112 Pamphlet
Patent Document 3: Japanese Patent Laid-open Publication No. 2007-109457
In this plasma processing apparatus, after plasma is ignited by locating the slugs of the tuner at an ignition position where the plasma ignition easily occurs, the slugs are moved to a matching position where the impedance is matched. Further, if a plasma condition such as a pressure, a power, a gas type or the like is changed in the state that the impedance is matched, the slugs are moved to a re-matching position where the impedance is matched under the new condition. At this time, conventionally, the slugs are moved through a matching track in which the moving distance of the slugs is shortest, only based on the reflection coefficient regardless of the plasma state. Thus, the matching track of the slugs may include an area where plasma emission intensity is low, and, in such a case, plasma may be extinguished or an operation may become unstable.